Acetylated tetralins and indanes are important musk fragrances widely used in the fragrance industry. Very well known among them is the acetylated tetralin 6-acetyl-1,2,3,4-tetrahydro-1,1,2,4,4,7-hexamethylnapthalene (the structure of which is shown in FIG. 1), which is also known by the trade names Tonalide, Tetralide and Fixolide. This musk can be prepared by acetylation of the corresponding tetrahydronaphthalene 1,2,3,4-tetrahydro-1,1,2,4,4,7-hexamethylnaphthalene which is sometimes also referred to as 1,1,3,4,4,6-hexamethyltetralin (and which is referred to herein as "HMT" for brevity), by the reaction shown in FIG. 2. The musk is thus referred to herein as "acetyl-HMT" for brevity.
Chlorinated solvents are conventionally used in such processes, but these are now considered environmentally undesirable. The present applicants have attempted to produce acetyl-HMT by acetylation of HMT using more environmentally acceptable saturated hydrocarbon solvents, particularly cyclohexane or other alicyclic solvents. However, they found that although this is possible on a small laboratory scale, it was difficult in practice on a larger scale, because a complex between the Lewis acid used and the ketonic product was formed which is insoluble or difficultly soluble in the non-halogenated solvent, with the consequence that the whole mixture is very difficult to stir or may even set solid. In large scale reaction vessels this may lead to an uneven temperature distribution and local overheating in the reaction mixture, and even damage to the stirring equipment. Also, the reaction vessel cannot or only with difficulty be emptied. Attempts to maintain the reaction mixture in liquid state by heating lead to unsatisfactory results due to poorer selectivity (either by side reaction, or by further reaction of the product), loss of volatile materials and unpredictable resolidification. Nevertheless, to prevent the difficulties caused by solidification of the reaction mixture, it is important that the reaction mixture either remains mobile at the reaction temperature or may be maintained in a mobile state by a moderate rise in temperature, preferably near or after completion of the addition of all components of the reaction mixture.
It has now been found that by performing an acylation reaction on a mixture of HMT and one or more substituted indanes and/or substituted acylindanes, it is possible successfully to perform the acylation reaction using a non-halogenated saturated hydrocarbon solvent, and so to produce acyl-HMT in a more environmentally acceptable way than has hitherto been feasible in practice.